extern "C"

[George2]

Hello everyone,


Just to confirm what is the most correct way beyond a just working function.

1.

We need to add extern "C" to both variable/function definition/declaration? Or only need to add to the variable/function declaration?

2.

How about extern? Declaration only or both declaration and definition are required?

BTW: previously, I only add to declaration, but after reading more and more code which add to both declaration and definition, I come to here to ask this question.


thanks in advance,
George

[weaknessforcats]

Do you know what extern "C" is for??

[RedSon]

Do you know what extern "C" is for??

I think we can all answer that for the OP...

[George2]

Sure, weaknessforcats . Used for stopping mangling name for function and variable, any comments to my original question?

Do you know what extern "C" is for??

regards,
George

[George2]

Thanks RedSon!


Any comments to my post #4?

I think we can all answer that for the OP...

regards,
George

[RedSon]

Sure, weaknessforcats . Used for stopping mangling name for function and variable, any comments to my original question?





regards,
George

Actually it is more complicated then that. It's true that "extern c" does prevent mangling but that is a side effect of c linking. What compiler are you using?

[hdanw]

Use the extern keyword so that they can be linked outside of its normal frame of reference.

The "c" tells the compiler how to push information on the stack.

The keyword should be added to the Definition of functions, so they can be called by other files.

And declaring extern on a declaration means that the actual declaration exists outside of this file. And should be visible to all linked object files.

// myfile.cpp
int globalint

extern myfunc()
{
  // do some stuff
}

// myfile.hpp

extern int globalint;
extern myfunc();

// other.cpp

#include "myfile.hpp"

globalint = 10;
myfunc();

[George2]

Thanks RedSon,


I am using Visual Studio 2008 to develop C/C++. Any advice about whether to add extern "C" to declaration only or both declaration and definition?

Actually it is more complicated then that. It's true that "extern c" does prevent mangling but that is a side effect of c linking. What compiler are you using?

regards,
George

[George2]

Thanks hdanw,


Please review whether I am correct.

1. We should add extern to declaration, so what we know we are referring a function/variale from other compile unit;
2. We should also add extern to definition so that this function/variale is visible to other compile unit. In this case, extern is on the other side of static.

Right?


regards,
George

Use the extern keyword so that they can be linked outside of its normal frame of reference.

The "c" tells the compiler how to push information on the stack.

The keyword should be added to the Definition of functions, so they can be called by other files.

And declaring extern on a declaration means that the actual declaration exists outside of this file. And should be visible to all linked object files.

// myfile.cpp
int globalint

extern myfunc()
{
  // do some stuff
}

// myfile.hpp

extern int globalint;
extern myfunc();

// other.cpp

#include "myfile.hpp"

globalint = 10;
myfunc();

[weaknessforcats]

I am using Visual Studio 2008 to develop C/C++. Any advice about whether to add extern "C" to declaration only or both declaration and definition?

I just did a Google on "extern "C" " and got 309,000 hits.

Maybe you could read a few of these pages. What you are asking is covered in any C++ reference material.

You are supposed to post questions when you are stuck in the code and not as a substitute for doing your own research.

Next:

All function prototypes and definitions have a default storage class specifier of extern. That is, the functions have external linkage. This means you do not need to specify extern. You can override the default with static. However, using the "C" qualifier will require the extern spcifier.

[RedSon]

Educate yourself ...

[George2]

Thanks weaknessforcats ,


I think we should add extern "C" to both declaration and definition, right?

http://www.glenmccl.co m/ansi028.htm

I just did a Google on "extern "C" " and got 309,000 hits.

Maybe you could read a few of these pages. What you are asking is covered in any C++ reference material.

You are supposed to post questions when you are stuck in the code and not as a substitute for doing your own research.

Next:

All function prototypes and definitions have a default storage class specifier of extern. That is, the functions have external linkage. This means you do not need to specify extern. You can override the default with static. However, using the "C" qualifier will require the extern spcifier.

regards,
George

[George2]

Hi RedSon,


I do not think the same in the MSDN page is good enough. When demonstrating extern, it should use more than one cpp files (more than one compile unit) right?

// specifying_linkage1.cpp
int i = 1;
void other();

int main() {
   // Reference to i, defined above:
   extern int i;
}

void other() {
   // Address of global i assigned to pointer variable:
   static int *external_i = &i;

   // i will be redefined; global i no longer visible:
   // int i = 16;
}

Educate yourself ...

regards,
George

[RedSon]

Hi RedSon,


I do not think the same in the MSDN page is good enough. When demonstrating extern, it should use more than one cpp files (more than one compile unit) right?

regards,
George

The point of providing the link was to give you a starting point into your investigation into the extern keyword. Did you follow any of the links in the see also section?

[George2]

Sure, RedSon.


Any comments to my post #13? I think to demonstrate the usage of extern, we should use more than one compile unit (cpp file) to demonstrate, right?

The point of providing the link was to give you a starting point into your investigation into the extern keyword. Did you follow any of the links in the see also section?

regards,
George